What has been your experience of the accuracy of max hill gradients using route mapping software?
I did a 12.8mile TT at the Weekend and a RWGPS plotted route said the max climb was 5.6% but the garmin recorded 13.2%. It tends to be closer on normal rides perhaps the garmin can't record fast enough a some speeds, at low speeds the garmin tends to be slightly under IIRC.
I find it very disconcerting that if you plot a long route, a stated 10% max gradient could in reality be a 20% max gradient.
I have a feeling that if you plot it as just a few miles which include your selected climb, it might show it as steeper than it would as part of a longer route.
The best thing to do is just use it as your only reference as @figbat says.
How else are you going to measure it?
You could get off your bike and use a spirit level or app. A bit of a faff.
Or you could look down at your GPS at the steepest part. I've tried this but at the steepest bit of a hill I tend to be unable to focus. And anyway, how reliable is the instantaneous GPS readings anyway?